Embodiments of the present invention relate to measuring physical properties of a fluid containing dispersed solid particulate material and having a yield stress. Merely by way of example, such fluids containing dispersed solid particulate material may comprise drilling muds/drilling fluids, which muds/fluids exhibit sag.
The rheological properties of liquids containing a dispersed solids particulate phase are well-studied. Such materials usually have a yield stress and are non-Newtonian in nature.
However it is difficult to measure the rheological properties by traditional rheometric methods. Furthermore, even if measurements of such materials are made it is usually the apparent viscosity which is measured, i.e., the shear stress divided by the shear rate.
However, for non-Newtonian fluids the apparent viscosity has little rheological importance, and is not useful for deriving other physical properties of the fluid.
Drilling muds, employed to assist in the drilling of oil and gas reservoirs are examples of such non-Newtonian fluids. The drilling muds/fluids, like many other fluids containing dispersed solid particulate material, typically have a yield stress, so that any solid particles do not sediment out of the mud/fluid when the mud/fluid is static. However, even with this property, some sedimentation still occurs in drilling muds/fluids in a process that is termed “sag” or “sagging.”
A number of methods have been proposed to measure the sag tendency of drilling muds. All of the methods require the attention of a skilled operator, and all use measurement of the vertical density gradient in the drilling mud/fluid to quantify the sag. Although the methods have been described as applying to drilling muds/fluids, the methods of measuring sag are also applicable to other particulate containing fluids, e.g., fluids for cuttings re-injection (“CRI”) or the like.
Hydrocarbons such as oil or gas are extracted from underground reservoirs through a well bored into the rock formation by means of a drill bit connected to a drill string or coil tubing. Rock cuttings are removed from the bit by means of a drilling fluid or mud that is pumped down the pipe or coil tubing. This mud/fluid returns the cuttings generated by the drill bit as it bores through the rock to the surface via the annulus formed between the outside of the drill string and the inner-wall of the wellbore created in the formation. At the surface, cuttings that are larger than about 150 microns are removed from the drilling mud/fluid by shale-shaker screens. In order to maintain wellbore stability, the pressure of the mud/drilling fluid at any depth in the wellbore should exceed the pressure of liquid in the pore space of the formation (known as the “pore pressure”) so as to prevent flow of formation fluids into the wellbore.
To provide for maintaining a pressure in the wellbore that exceeds the pore pressure, generally the drilling mud is “densified” by the addition of a finely divided weighting agent, typically barite particles, but often other minerals or a combination of minerals may be used, where the weighting agent density exceeds the un-weighted mud density. The weighting agent is added to the mud in a quantity that is determined to be sufficient to achieve the desired mud density. The lower limit of the particle size of the weighting agent is selected such that it does not affect the mud's rheology or damage the permeability of the formation in a reservoir section. An upper limit of the particle size is given by the mesh size of the shale shaker screens used to remove cuttings from the circulated mud. Merely by way of example, the size may lie in the 10 micron to 100 micron range.
Being of this size, the particles are not suspended by Brownian motion and so are prone to sediment under the influence of gravity; this process is termed “sag.” Sag causes variations in the density of the drilling mud, both in the borehole, and as it emerges at the surface. Sag can lead to many problems; e.g., well-control, fracturing of the uncased formation with lost circulation, and the sticking of the drill pipe or other down hole hardware in beds of sagged weighting material.
For drilling muds in a cased or uncased hole the yield stress is a factor in the tendency of wireline logging tools and cables to stick in the hole. Thus the invention proposes a novel means of quantifying sag in general, and also a means that relates specifically to the sticking of wireline hardware.
Additionally, in cuttings re-injection (CRI), rock cuttings from a well are separated from the drilling mud via a shaker screen and ground to a specified maximum size (typically 300 micron) and slurried with seawater and usually a polymer to give a fluid. This fluid is pumped downhole to fracture a rock formation, hence disposing of the cuttings. The slurry viscosity is required to be as small as possible to minimize the friction pressure drop in the pipework and the fracture, but large enough to suspend the cuttings during the transit time from the surface pumps to the fracture. Careful design and sag measurement of CRI fluids is required to avoid unwanted sedimentation of the cuttings.
Thus, a method of testing such fluids to produce useful information as to their physical properties would be desirable.